We have a number of Intel 82599EB 10-Gigabit dual port NICs running in a number of servers. On a couple of these servers, we are seeing very strange behaviour where when connecting via a Brocade 1G/10G switch from a 1G server we are getting line speed RX (113MB/s) but only a fraction of this TX, for example:

We have another server with the same card and that behaves as expected (line speed both TX and RX). As far as I can tell all network configs are the same on both boxes. We've tried different switch, cable, OS and server hardware.